Shock absorbing bumper

ABSTRACT

An energy absorbing support unit for an automobile bumper bar, the support unit including a pair of telescopically related tube members adapted for disposition between the bumper bar and the vehicle chassis, an orificed piston rigidly attached to one tube member and adapted to effect throttled flow of an incompressible medium situated in a chamber in the other tube member to effect energy absorption, a free piston slidably mounted in the one tube member and resiliently biased toward a normal position corresponding to a fully extended position of the one tube member, the free piston urging reverse flow of the incompressible medium to provide a self-restoring preload on the one tube member, and a reciprocating valve member on the free piston movable between an open position and a closed position partially obstructing the piston orifices to increase the rate of energy absorption. The free piston, in the normal position thereof, engages the valve member to positively locate the latter in the open position whenever the one tube member is in its fully extended position.

United States Patent [191 Thorsby et a1.

1 1 SHOCK ABSORBING BUMPER [75] lnventors: Claude A. Thorsby, Saginaw;Robert L. White, Frankenmuth, both of [51] Int. Cl. ..B6lf 19/02, B6lf19/04, Fl6f 9/30 [58] Field of Search ..l88/1 B, 1 C, l D, l R, 268,188/317; 213/43, 221, 222; 267/134, 135,

[56] References Cited UNITED STATES PATENTS 1,648,394 11/1927 Gates..295/85 3,053,526 9/1962 Kendall ..l88/268 X 3,379,317 4/1968 Carlson..267/1 34 X 4/1968 Stasieluk ..l88/268 X Feb. 6, 1973 PrimaryExaminer-Drayton E. Hoffman Assistant Examiner-Howard BeltranAttorney-W. E. Finken et a1.

[57] ABSTRACT An energy absorbing support unit for an automobile bumperbar, the support unit including a pair of telescopically related tubemembers adapted for disposition between the bumper bar and the vehiclechassis, an orificed piston rigidly attached to one tube member andadapted to effect throttled flow of an incompressible medium situated ina chamber in the other tube member to effect energy absorption, 21 freepiston slidably mounted in the one tube member and resiliently biasedtoward a normal position corresponding to a fully extended position ofthe one tube member, the free piston urging reverse flow of theincompressible medium to provide a self-restoring pre load on the onetube member, and a reciprocating valve member on the free piston movablebetween an open position and a closed position partially obstructing thepiston orifices to increase the rate of energy absorption. The freepiston, in the normal position thereof, engages the valve member topositively locate the latter in the open position whenever the one tubemember is in its fully extended position.

4 Claims, 5 Drawing Figures PAIENTEDFEB '6 ms 7 3.715114 SHEET 2 BF 2INVEN TORS HTTOR Y SHOCK ABSORBING BUMPER The invention relatesgenerally to energy absorbing devices of the telescopically collapsibletype and more particularly to an improved energy absorber having valvemeans adapted to vary the resistance to collapse and hence the rate ofenergy absorption from a minimum at the instant collapse commences to amaximum at some point in time thereafter.

In the automotive field, it has been suggested that certain beneficialresults are obtainable by supporting the collision bumper bar of thevehicle on the chassis of the vehicle by means of energy absorbing unitswhich cushion impacts on the bar and absorb the energy of impact beforecontact is made with the body sheet metal. With respect to suchinstallations it has also been suggested that further beneficial resultsmight be obtained by varying the rate of energy absorption from aminimum at the instant of impact to a maximum at some point in timethereafter. Numerous designs for energy absorber units processing thischaracteristic have been suggested. Most, however, are unsatisfactoryfor any number of reasons including undue complexity and performanceunreliability. An energy absorber according to this invention representsan improvement over known similar devices having the characteristicdescribed in that it is relatively simple in construction and veryreliable in operation.

Accordingly, the primary feature of this invention is that it providesan improved energy absorber having a variable rate of energy absorption,the energy absorber being particularly suitable for supporting acollision bumper bar on an automobile chassis. Another feature of thisinvention is that it provides an improved energy absorber of thetelescopically collapsible oleo-strut type including valve means forvarying the rate of energy absorption and positioning means adapted topositively locate the valve means in a predetermined posi tion insuringminimum initial resistance to telescopic collapse from a fully extendedcondition of the energy absorber. Still other features of this inventionreside in the provision, in an energy absorber of the oleo-strut typeincluding a working piston for throttling fluid flow to absorb energyand a free piston resiliently biased in one direction to preload theenergy absorber, of a valve means on the working piston movable betweenopen and closed positions to vary the rate of energy absorption and inthe provision of means on the free piston and on the valve meansengageable in the fully extended condition of the energy absorber topositively locate the valve means in the open position.

These and other features of this invention will be readily apparent fromthe following specification and from the drawings wherein:

FIG. 1 is a perspective view of the forward portion of an automobileframe having a collision bumper'structure supported thereon by a pair ofimproved energy absorbing support units according to this invention;

FIG. 2 is an enlarged sectional view taken generally along the planeindicated by lines 2-2 in FIG. 1 and showing the support unit in a fullyextended condition;

. FIG. 3 is a view similar to FIG. 2 but showing the support unit in afully collapsed condition;

FIG. 4 is an enlarged view of a portion of FIG. 1 showing a modifiedvalve construction in an open position corresponding to the fullyextended condition of the support unit; and

modified valve construction in a closed position corresponding to thefully collapsed condition of the support unit.

Referring now to FIG. 1 of the drawings, there shown in simplified formis an automobile frame designated generally 10 including a pair oflongitudinally extending side rails 12 and 14, the side rails beinginterconnected at the forward end of the frame by a rigid cross member16. A collision bumper structure 18, including a bumper bar 20, issupported on the frame by a pair of identical energy absorbing supportunits for absorbing shocks according to this invention and designatedgenerally 22, each support unit being rigidly attached to a respectiveone of the frame rails 12 and 14 by a front bracket 24 and a rearbracket 26 each bolted to the frame rails. It will be understood, ofcourse, that rather than the separate frame illustrated herein, thesupport units according to this invention are equally well adapted foruse with unibody vehicle constructions.

Referring now to FIGS. 2 and 3 and describing only one support unit 22,the latter includes a cylindrical mounting tube 28 closed at one end byan end plate 30. The mounting tube has a large diameter portion 32adjacent the end plate 30 and a small diameter portion 34 extending fromthe open end of the mounting tube, the small and large diameter portionsbeing interconnected by an annular shoulder portion 36. An attachingflange 38 is butt welded to the small diameter portion of the mountingtube and is adapted for rigid connection to the bracket 24, as by bolts40, while the end plate 30 is adapted by conventional means, not shown,for rigid connection to the bracket 26.

A cylindrical support tube 42 is closely slidably received in the smalldiameter portion 34 of the mounting tube 28 and includes an annularabutment portion 44 having an outside diameter generally equal to theinside diameter of large diameter portion 32 of the mounting tube.Accordingly, the support tube is slidably supported on the mounting tubewith substantial lateral rigidity for axial telescopic collapse from afully extended position corresponding to the fully extended condition ofthe support unit, FIG. 2, defined by engagement between the annularabutment 44 and shoulder portion 36 to a fully collapsed positioncorresponding to the fully collapsed condition of the support unit, FIG.3, wherein the inboard end of the support tube is disposed adjacent endplate 30. The outboard end of the support tube is sealingly closed by amounting flange 46 butt welded to the support tube at 48, the flange 46being adapted for rigid connection to the bumper bar 20.

As seen best in FIGS. 2 and 3, a circular working piston 50 having aplurality of orifices 52 arranged in a circular pattern around thecenter of the piston is rigidly attached to the inboard end of supporttube 42, as by a butt weld at 54. A free piston 56 is slidably disposedwithin the support tube 42 and carries a sealing ring 58, the piston andsealing ring dividing the support tube into two variable volurnechambers 60 and Referring again to FIGS. 2 and 3, the chamber 66 has amaximum volume in the fully extended position of the support tube 42 andis completely filled with a flowable and incompressible medium 68. Theincompressible medium may be a liquid or any one of many commerciallyavailable silicon compounds and is adapted for throttled flow fromchamber 66, through the orifices 52, into chamber 62 in response totelescopic collapse of the mounting tube 42 from the fully extendedposition toward the fully collapsed position. The throttling of themedium 68 through the orifices 52 effects dissipation or absorption ofenergy transferred to the support unit by an impact on the outboard endof the support tube 42.

The throttled incompressible medium, of course, exerts pressure on thefree piston 56 urging the latter leftward relative to the mounting tube42 thereby to expand chamber 62 while simultaneously reducing the volumeof chamber 60. The chamber 60 is filled with a compressible medium 70,preferably a gas, which increasingly resists relative leftward movementof the free piston 56 thereby to generate on the latter a rightwardlydirected force urging reverse flow of the incompressible medium from thechamber 62 into the chamber 66. The compressible medium 70 is normallyintroduced into the chamber 60 under substantial initial pressure sothat the free piston 56 is forced rightwardly relative to the supporttube to a normal position, FIG. 2, juxtaposed to working piston 50during which movement substantially all of the incompressible medium 68is evacuated from the chamber 62 into the chamber 66, the pressureexerted on the medium 68 by the compressible medium 70 through the freepiston effecting relative extension of the support tube to the fullyextended position under a preload depending upon the initial pressure ofthe compressible medium 70.

As best seen in FIGS. 2 and 3, a valve member 72 is provided to effectthe desirable result of relatively low resistance to initial telescopiccollapse of the support tube and higher resistance to continuedcollapse, the valve member including a cylindrical shank portion 74having a head portion 76 at one end and a restricting portion 78 at theother end. The shank portion 74 of the valve member is slidablysupported in an aperture 80 in the working piston 50 with the headportion 76 on one side of the piston and the restricting portion 78 onthe other for reciprocative movement relative to the piston between anopen position, FIG. 2, wherein the orifices 52 are unobstructed and aclosed position, FIG. 3, wherein the restricting portion 78 partiallyobstructs the orifices 52 to reduce the effective area of the latter.When the free piston 56 is in its normal position, FIG. 2, correspondingto the fully extended condition of the support unit, the head portion 76of the valve member 72 is captured between the working piston and theface of the free piston so that the valve member is held in the openposition.

The differential area of the valve member 72 exposed to the prevailingpressure in the chamber 66 cooperates with the medium 68 to move thevalve member from the open position to the closed position in responseto telescopic collapse of the support tube 42. More particularly, thepiston 50, in response to a collapsing impact on the support tube 42,places the medium 68 in chamber 66 under a pressure exceeding thepressure in chamber 62 thereby initiating flow of the medium 68 throughthe unobstructed orifices 52 so that collapsing movement of the supporttube begins against minimum resistance. The throttled working mediumentering chamber 62 forces the free piston 56 leftward into the supporttube thereby freeing the head portion 76 and the valve member to permitmovement of the latter from the open to the closed position. The higherpressure in chamber 66 acting on the differential area of the valvemember, i.e., the cross sectional area of the valve member shank portion74, which is ex posed to high pressure on one side and a lower pressureon the other side, initiates movement of the valve member from the openposition toward the closed position so that at some point in time afterthe initial impact when the valve member reaches the closed positionwith the restricting portion seated on the face of the working pistonand partially obstructing the orifices 52, the incompressible medium isthrottled through the substantially reduced orifices thereby effectingan increase in the resistance to further telescopic collapse and hencean increase in the rate of energy absorption.

After the collapse causing impact on the support unit ceases, thecompressed medium acting on the free piston 56 initiates reverse flow ofthe incompressible medium 68 to telescopically extend the support tube,the reverse flow being the result of the pressure in chamber 62 whichnow exceeds the pressure in chamber 66. The initial pressure at whichthe compressible medium 70 is introduced into chamber 60 ispredetermined to insure that after impact the free piston will returnvirtually to its normal position thereby insuring that substantially allof the medium 68 returns to chamber 66 and, consequently, that thesupport tube 42 returns to the extended position. Return of the freepiston to its normal position also insures that the valve member 72 willbe in the open position to provide minimum resistance to anothercollapse cycle of the support unit. More particularly, as the supporttube 42 begins to extend from the fully collapsed position, FIG. 3, theprevailing pressure in chamber 62 acting on the same differential areaof the valve member but in a reverse direction effects movement of thevalve member 72 from the closed position, FIG. 3, back to the openposition, FIG. 2, some time before the support tube reaches the fullyextended position. That such will be the case is, however, positivelyinsured by the free piston 56, the face of which engages the headportion 76 of the valve member to locate the latter in the open positionwhen the free piston and the support tube respectively reach the normaland extended positions. In addition, with the head portion 76 capturedbetween the free piston and the working piston there is no possibilityof the valve member 72 inadvertently moving to the closed position as aresult of vibrations or the like.

Referring now to FIGS. 4 and 5, thereshown is a modified or compositevalve member designated generally 82 adapted to effect obstruction ofthe orifices 52 in the working piston 50 in two stages as opposed to thesingle stage obstruction described hereinbefore with respect to valvemember 72. More particularly, the modified valve member 82 includes acylindrical collar 84 having a retaining flange 86 at one end thereofand an enlarged restricting flange 88 at the other end thereof. Thecollar is slidably supported in a central aperture 90 in the piston 50with the retaining flange on one side of the latter and the restrictingflange on the other. A valve element 92 is slidably supported on thecollar 84 and includes a head portion 94 outboard of the restrictingflange 86 and a restricting portion 96 outboard of the restrictingflange 88.

As seen best in FIG. 4, when the free piston 56 is in its normalposition corresponding to thefully extended position of the support tube42, the face of the free position engages the head portion 94 of themodified valve member 82 to positively locate the latter in an openposition, FIG. 4, wherein both the restricting flange 88 and therestricting portion 96 are situated remote from the orifices 52 so thatthe latter are unobstructed. With the head portion and retaining flangethus captured, inadvertent movement of the modified valve member out ofthe open position is effectively foreclosed.

When the support tube 42 is collapsed under impact from the fullyextended position, the incompressible medium 68 is throttled through theunobstructed orifices to effect energy absorption at a first, minimumrate and thereafter displaces the free piston 56 leftwardly with respectto the support tube to permit unitary leftward movement of the collar 84and the valve element 92 relative to the working piston 50. Suchmovement corresponds to movement of the modified valve member from theopen position to a first closed position, not shown, wherein restrictingflange 88 seats against piston 50 and partially obstructs orifices 52.Such partial obstruction effects a first increase in the resistance totelescopic collapse of the support tube 42 and hence a first increase inthe rate of energy absorption.

Continued telescopic collapseof the support tube effects furtherleftward bodily movement of the valve element 92 relativeto the piston50 and the collar 84 until the restricting portion 96 seatingly engagesthe restricting flange 88, FIG. 5, thus defining a second closedposition of the modified valve member 82. In the second closed positionthe restricting portion 96 further obstructs the orifices 52 thereby toeffect a second and higher rate of energy absorption. When the collapsecausing impact on the support tube 42ceases, the compressed medium 70 inthe chamber 60effects reverse flow of the incompressible medium 66 untilthe free piston 56 engages the head portion 94 to once again insurepositive location of the modified valve member in the open position. V

Having thus described the invention, what is claimed is:

1. In an energy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneof said tube members and having a plurality of orifices therein and afree piston slidably disposed in said one tube member, wherein anincompressible working medium in the other of said tube members isthrottled through said piston orifices during relative telescopiccollapse between said tube members to effect energy absorption andwherein the. throttled incompressible medium exerts a pressure on saidfree piston to compress a compressible working medium in said one tubemember on the opposite side of said free piston thereby to generate arestoring force urging relative extension between said tube members to afully extended condition, the improvement comprising, a valve memberdisposed on said orificed piston for reciprocative movement relative tothe latter between. an open position wherein said orifices areunobstructed and a closed position wherein said orifices are partiallyobstructed for effecting increased resistance to flow of saidincompressible working medium and consequently energy absorption at anincreased rate, means on said valve member cooperable with saidincompressible working medium to effect movement of said valve memberfrom the open position to the closed position in response to relativecollapse between said tube members from the fully extended condition forincreasing the rate of energy absorption after initiation of collapse,and means on said valve member and on said free piston cooperable toeffect positive location of said valve member in the open positionthereof when said tube members achieve the fully extended conditionthereby to ensure minimum resistance to initial telescopic collapsebetween said tube members from the fully extended condition.

2. In an energy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneof said tube members and having a plurality of orifices therein andafree piston slidably disposed in said one tube member, wherein anincompressible working medium in the other of said tube members isthrottled through said piston orifices during relative telescopiccollapse between said tube members to effect energy absorption andwherein the throttled incompressible medium exerts pressure on said freepiston to move the latter relative to said one tube member from a normalposition corresponding to a fully extended condition of said tubemembersthereby to compress a compressible medium in said one tube member on theopposite side of said free piston for generating a restoring forceurging relative extension between said tube members to the fullyextended condition, the improvement comprising, a valve member having ahead portion on one end and a restricting portion on the other end,means mounting said valve member on said orificed piston with said headportion disposed on one side of said orificed piston and saidrestricting portion disposed on the other side for reciprocativemovement relative to said orificed piston between an open positionwherein said orifices are unobstructed and a closed position whereinsaid orifices are-partially obstructed for effecting increasedresistance to flow of said incompressible working medium andconsequently energy absorption at an increased rate, means on said valvemember cooperable with said incompressible working medium to effectmovement of said valve member from the open position to the closedposition in response to relative collapse between said tube members.from the fully extended condition for increasing the rate of energyabsorption after initiation of collapse, and means on said free pistonengageable on said valve member head portion in the normal position ofsaid free piston to positively locate said valve member :in the openposition thereby to ensure minimum resistance to initail telescopiccollapse between said tube members from the fully extended condition.

3. In an energy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneof said tube members and having a plurality of orifices therein and afree piston slidably disposed in said one tube member, wherein anincompressible working medium in the other of said tube members isthrottled through said piston orifices during relative telescopiccollapse between said tube members to effect energy absorption andwherein the throttled incompressible medium exerts pressure on said freepiston to move the latter relative to said one tube member from a normalposition corresponding to a fully extended condition of said tubemembers to compress a compressible medium in said one tube member on theopposite side of said free piston for generating a restoring forceurging relative extension between said tube members to the fullyextended condition, the improvement comprising, a composite valve memberhaving a head portion and a first restricting portion and a secondrestricting portion bodily movable relative to said first restrictingportion, means mounting said composite valve member on said orificedpiston with said head portion on one side of said orificed piston andeach of said first and said second restricting portions on the otherside for sequential reciprocative movement relative to said orificedpiston between an open position wherein said orifices are unobstructedand a first closed position wherein said second restricting portionpartially obstructs said orifices to effect increased resistance to flowof said incompressible working medium and a second closed positionwherein said first restricting portion further obstructs said orificesto effect further resistance to flow of said incompressible medium,means on said composite valve member cooperable with said incompressiblemedium to effect sequential movement of said composite valve member fromthe open position to the first closed position and to"the second closedposition in response to relative telescopic collapse between said tubemembers from the fully extended condition thereby for effectingsequential increase in the resistance to flow of said incompressiblemedium and consequently energy absorption at an increased rate after theinitiation of collapse, and means on said free piston engageable on saidcomposite valve member head portion in the normal position of said freepiston to positively locate said composite valve member in the openposition thereof thereby to insure minimum resistance to initialtelescopic collapse between said tube members from the fully extendedcondition.

4. In an energy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneof said tube members and having an orifice therein and a free pistonslidably disposed in said one tube member, wherein an incompressibleworking medium in the other of said tube members is throttled throughsaid piston orifice during relative telescopic collapse between saidtube members to effect energy absorption and wherein the throttledincompressible medium exerts a pressure on said free piston to effectbodily shiftable movement thereof relative to said one tube member andcompression of a compressible working medium in said one tube member onthe opposite side of said free piston thereby to generate a restoringforce urging relative extension between said tube members to a fullyextended condition, the improvement comprising, restricting meansassociated with said piston orifice and adapted for bodily shiftablemovement relative to said piston between a first position correspondingto the fully extended condition of said tube members wherein maximumflow of said working medium is permitted and a second positioncorresponding to at least a partially collapsed conditionof said tubemembers wherein said position orifice is partially obstructed thereby torestrict the flow of said working medium, means on said free piston andon said restricting means adapted to positively locate said restrictingmeans in the first position thereof when said tube members are in thefully extended condition, and means on said free-piston and on saidrestricting means operative to effect unitary movement of said freepiston with said restricting means duringmovement of the latter from thefirst to the second position thereof in response to collapse of saidtube members from the fully extended condition.

1. In an energy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneof said tube members and having a plurality of orifices therein and afree piston slidably disposed in said one tube member, wherein anincompressible working medium in the other of said tube members isthrottled through said piston orifices during relative telescopiccollapse between said tube members to effect energy absorption andwherein the throttled incompressible medium exerts a pressure on saidfree piston to compress a compressible working medium in said one tubemember on the opposite side of said free piston thereby to generate arestoring force urging relative extension between said tube members to afully extended condition, the improvement comprising, a valve memberdisposed on said orificed piston for reciprocative movement relative tothe latter between an open position wherein said orifices areunobstructed and a closed position wherein said orifices are partiallyobstructed for effecting increased resistance to flow of saidincompressible working medium and consequently energy absorption at anincreased rate, means on said valve member cooperable with saidincompressible working medium to effect movement of said valve memberfrom the open position to the closed position in response to relativecollapse between said tube members from the fully extended condition forincreasing the rate of energy absorption after initiation of collapse,and means on said valve member and on said free piston cooperable toeffect positive location of said valve member in the open positionthereof when said tube members achieve the fully extended conditionthereby to ensure minimum resistance to initial telescopic collapsebetween said tube members from the fully extended condition.
 1. In anenergy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneof said tube members and having a plurality of orifices therein and afree piston slidably disposed in said one tube member, wherein anincompressible working medium in the other of said tube members isthrottled through said piston orifices during relative telescopiccollapse between said tube members to effect energy absorption andwherein the throttled incompressible medium exerts a pressure on saidfree piston to compress a compressible working medium in said one tubemember on the opposite side of said free piston thereby to generate arestoring force urging relative extension between said tube members to afully extended condition, the improvement comprising, a valve memberdisposed on said orificed piston for reciprocative movement relative tothe latter between an open position wherein said orifices areunobstructed and a closed position wherein said orifices are partiallyobstructed for effecting increased resistance to flow of saidincompressible working medium and consequently energy absorption at anincreased rate, means on said valve member cooperable with saidincompressible working medium to effect movement of said valve memberfrom the open position to the closed position in response to relativecollapse between said tube members from the fully extended condition forincreasing the rate of energy absorption after initiation of collapse,and means on said valve member and on said free piston cooperable toeffect positive location of said valve member in the open positionthereof when said tube members achieve the fully extended conditionthereby to ensure minimum resistance to initial telescopic collapsebetween said tube members from the fully extended condition.
 2. In anenergy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneoF said tube members and having a plurality of orifices therein and afree piston slidably disposed in said one tube member, wherein anincompressible working medium in the other of said tube members isthrottled through said piston orifices during relative telescopiccollapse between said tube members to effect energy absorption andwherein the throttled incompressible medium exerts pressure on said freepiston to move the latter relative to said one tube member from a normalposition corresponding to a fully extended condition of said tubemembers thereby to compress a compressible medium in said one tubemember on the opposite side of said free piston for generating arestoring force urging relative extension between said tube members tothe fully extended condition, the improvement comprising, a valve memberhaving a head portion on one end and a restricting portion on the otherend, means mounting said valve member on said orificed piston with saidhead portion disposed on one side of said orificed piston and saidrestricting portion disposed on the other side for reciprocativemovement relative to said orificed piston between an open positionwherein said orifices are unobstructed and a closed position whereinsaid orifices are partially obstructed for effecting increasedresistance to flow of said incompressible working medium andconsequently energy absorption at an increased rate, means on said valvemember cooperable with said incompressible working medium to effectmovement of said valve member from the open position to the closedposition in response to relative collapse between said tube members fromthe fully extended condition for increasing the rate of energyabsorption after initiation of collapse, and means on said free pistonengageable on said valve member head portion in the normal position ofsaid free piston to positively locate said valve member in the openposition thereby to ensure minimum resistance to initail telescopiccollapse between said tube members from the fully extended condition. 3.In an energy absorber of the collapsible type, including a pair oftelescopically related tube members and a piston rigidly attached to oneof said tube members and having a plurality of orifices therein and afree piston slidably disposed in said one tube member, wherein anincompressible working medium in the other of said tube members isthrottled through said piston orifices during relative telescopiccollapse between said tube members to effect energy absorption andwherein the throttled incompressible medium exerts pressure on said freepiston to move the latter relative to said one tube member from a normalposition corresponding to a fully extended condition of said tubemembers to compress a compressible medium in said one tube member on theopposite side of said free piston for generating a restoring forceurging relative extension between said tube members to the fullyextended condition, the improvement comprising, a composite valve memberhaving a head portion and a first restricting portion and a secondrestricting portion bodily movable relative to said first restrictingportion, means mounting said composite valve member on said orificedpiston with said head portion on one side of said orificed piston andeach of said first and said second restricting portions on the otherside for sequential reciprocative movement relative to said orificedpiston between an open position wherein said orifices are unobstructedand a first closed position wherein said second restricting portionpartially obstructs said orifices to effect increased resistance to flowof said incompressible working medium and a second closed positionwherein said first restricting portion further obstructs said orificesto effect further resistance to flow of said incompressible medium,means on said composite valve member cooperable with said incompressiblemedium to effect sequential movement of said composite valve member fromthe open position to the first closed position and to tHe second closedposition in response to relative telescopic collapse between said tubemembers from the fully extended condition thereby for effectingsequential increase in the resistance to flow of said incompressiblemedium and consequently energy absorption at an increased rate after theinitiation of collapse, and means on said free piston engageable on saidcomposite valve member head portion in the normal position of said freepiston to positively locate said composite valve member in the openposition thereof thereby to insure minimum resistance to initialtelescopic collapse between said tube members from the fully extendedcondition.